Kogalymavia (Metrojet), the Russian airline whose passenger jet broke up and crashed Saturday, has officially ruled out pilot error or "malfunction" as the cause of the crash, according to The Washington Post. But it's not clear how the airline reached that conclusion so quickly, though, because the black boxes and other wreckage haven't been fully analyzed, the Post pointed out. The airline's deputy general, Alexander Smirnov, said in a statement that the tragedy had to be the result of a "mechanical impact on the aircraft," meaning that the plane was tampered with or struck externally, for example. Though evidence is still limited, there are a few theories about the cause of the Russian plane crash that are being cited both by press and aviation experts.

The jet had just started its flight to St. Petersburg from an Egyptian resort when it broke into pieces and crashed in the Sinai Peninsula in Egypt on Saturday, according to CNN. All 224 people aboard the plane died, so the pressure to come up with an explanation has landed the airline under intense scrutiny. The airline's officials are just as baffled right now, stressing that planes don't just break into pieces, even if there is some kind of technical malfunction or pilot error, according to CNN. These four theories about what caused the crash are the most common, but there really isn't any more evidence for one than any of the others.

The Islamic State Shot It Down

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The Islamic State claimed responsibility for the downed plane via Twitter on Saturday, saying that the plane had been shot down in retaliation for Russian involvement in Syria, according to U.S. News & World Report. Director of National Intelligence James Clapper told reporters at the Defense One Summit that it would be strange for ISIS to shoot down a plane of civilians, due to the kind of international response it would elicit, but that it's not entirely unlikely given how "aggressive" the chapter of the Islamic State is in the Sinai Peninsula.

Louisa Loveluck reported that experts in the region say that, though the Sinai Peninsula chapter of ISIS is aggressive, it's largely unorganized and still "a guerrilla force" that more often carries out car bombings and drive-by attacks, according to The Daily Telegraph. Further, Loveluck reported that ISIS has claimed responsibility for dozens of attacks that later evidence showed it didn't perpetrate.

Metrojet also reportedly received a video that claimed to show the plane being shot down, but Metrojet officials said it appears to be fake, according to The New York Times. Plus, the plane was 31,000 feet in the air, so it would be extremely difficult to shoot it down when it's that high up. So far, there isn't any evidence that ISIS was involved in the crash, but it's not entirely impossible.

The airline has already said it doesn't believe technical problems were behind the crash. In response, press have pointed to a tail strike, which results when a plane's tail hits the runway, the plane suffered in 2001 and took three months to repair, BBC News reports. A poor tail strike repair reportedly caused Japan Airlines Flight 123 to crash in 1985, killing all but four people on board.

CNN aviation analyst Peter Goelz said this theory seems a pretty credible one: "Some sort of catastrophic failure, perhaps caused by an earlier maintenance problem," could have brought the plane down. He said a center fuel tank could have exploded and caused the plane to break apart in the way it did.

Pilot Error May Have Been A Factor

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Though the airline has also ruled out human error on the part of the pilots, Robert T. Francis, a former vice chairman of the National Transportation Safety Board in the United States, said the airline's statement seemed premature, according to the Times:

I am surprised that an airline manager, at the point that we are at in this investigation, would make a statement like that. Without the flight recorders having been read, and without more investigation of the fuselage, which is spread all over the place, I don’t think you can rule out anything.

Smirnov, the airline's deputy general, said the plane's airspeed slowed dramatically and that it dropped 5,000 feet one minute before it crashed. Smirnov didn't cite his source, but he said that sudden changes in speed and altitude just prior to the crash could mean that the pilots were having trouble controlling the plane, according to the Times.

Could There Have Been A Bomb On Board?

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The airline's conclusion that an "external influence" caused the crash has led some press to speculating whether "external influence" could include a bomb or another device that would cause the plane to break into pieces mid flight. Yet again, there is no real evidence that there was a bomb on board, but it's still a possibility.

Michael Clarke, a professor and director general of a national security think tank in the U.K. told BBC News that it's "much more likely" that there was a bomb on board than that ISIS fired a missile from the ground:

Early reports said that [the aircraft] split into two and that suggests a catastrophic failure, not a mechanical failure, but that suggests perhaps an explosion on board.

Analysis of debris will also help determine whether this is a more plausible theory. Right now, it seems too soon to rule anything out.